Friday, 3 February 2012

Tripped the lights - fantastic!

The breaker had gone a couple of times already, in hurry-up-and-make-breakfast mode. The total electricity is limited to 60 amps on the breaker board. The IH will use two or three kW to boil the kettle at full power, the boiler uses about two-and-half and is sometimes still going until 7 am when the cheap rates finish, and the oven uses a couple of kW, so that's well over 60 amps.

We were a bit surprised when the lights went off at about 10:30 on Saturday 14th January, when we weren't really using any power. In fact the lights didn't go off because none of them were on, and I'm not sure exactly how we found they had gone off. We had guests around, so perhaps we were trying to put the kettle on.

I'd noticed the panels were producing 8kW a little earlier, which was the most I had ever seen. The highest before that had been 7.3. I put the breaker back on again, and a few minutes later it went off again. Since then it has been fine, and the only reason I could think of was the panels producing too much power. I contacted the electrician and he said the same thing.

A couple of days later the solar suppliers came to explain how everything worked as they'd missed explanation day on Christmas eve. Actually it made a lot more sense to come after we'd been producing electricity for a month and had some actual data to look at on the display. A younger guy with a loud suit from the manufacturers, and the older man in company uniform from the installation company. They were impressed by how much we were producing and explained a bit more about the system. The power conditioners are rated at 4 kW each, and will limit their output if they are overheating or if too much power is coming from the panels to them. As the panels are rated at around 9.3 kW, when the conditions are good they will produce more than the power conditioners can take.

The display panel keeps track of how many minutes the power conditioners have been limiting output each day, and on some days it has been over 20 minutes, which they have never seen on a system at this time of year.

The electrician, in an email, said it should be be OK if we changed the breaker from a 60 amp to a 50 amp. This rather confused me, as 50 amps seems to be less than 60 amps. He explained that the 60 amp one is a special Chubu electric limiter for a total of 60 amps on either channel whereas the 50 amp is for 50 amps on both channels. The electricity seems to come in on two wires, 200 volts between them and each 100 volts from ground. Most electrical appliances inside the house use one side of this and ground to get 100 volts. I had thought it would be three phase but evidently not.

The solar power is going out at 200 volts, so even if it is at 8 kW, that's only going to be 40 amps (P = V I). However, the 60 amp fuse breaker adds up the current on each wire, and will look at that as 80 amps. If we change to a non-Chubu breaker, it will just work on the current, and 50 amps will be plenty.

What still confused me is why it had been fine before and since, and only tripped on that one morning. Why was 7.3 kW OK, but not 8kW? It seems that there is some variety in the actual current that the breaker will let through, and it looks like our 60 amp breaker will let through up to something like 75 amps. I asked if we could get an even weaker 60 amp breaker. The electrician laughed at my joke.

In terms of solar production, that day was a difficult case as it was partly cloudy. Otherwise it has either been sunny for the whole day, or cloudy for the whole day. On this day it had been a cold night, then a cloudy morning, so the panels and power conditioners must all have been cool and highly efficient. Then, suddenly, bright sunlight came out from the high sun, almost in the south. Maximum power attack.

This happened in the middle of January, but as the sun gets higher and more of its rays get onto the panels, there's going to be a significant amount of potential power lost because of the limit of the power conditioner. I'm not sure what can be done about this, but I wonder about a battery between the panels and power conditioner, and perhaps rewiring the panels into eights rather than sixes so they put out a higher voltage. The power conditioner will start producing AC as soon as the voltage gets over a threshold. The battery will charge when the panels are producing more voltage than it has, and discharge when the panels are producing less. As it discharges, its voltage will go down and the system should make the output more even, especially on those tricky days that are partly cloudy.

Obviously this is all just theoretical, and I wouldn't dream of actually adding a battery to my system as it would contravene the contract with the electrical company.